/*
 * Copyright 2002-2018 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.web.filter;

import java.io.IOException;
import java.util.Collections;
import java.util.Enumeration;
import java.util.List;
import java.util.Locale;
import java.util.Map;
import java.util.Set;
import javax.servlet.FilterChain;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletRequestWrapper;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpServletResponseWrapper;

import org.springframework.http.HttpRequest;
import org.springframework.http.HttpStatus;
import org.springframework.http.server.ServletServerHttpRequest;
import org.springframework.lang.Nullable;
import org.springframework.util.CollectionUtils;
import org.springframework.util.LinkedCaseInsensitiveMap;
import org.springframework.util.StringUtils;
import org.springframework.web.util.UriComponents;
import org.springframework.web.util.UriComponentsBuilder;
import org.springframework.web.util.UrlPathHelper;

/**
 * Extract values from "Forwarded" and "X-Forwarded-*" headers in order to wrap
 * and override the following from the request and response:
 * {@link HttpServletRequest#getServerName() getServerName()},
 * {@link HttpServletRequest#getServerPort() getServerPort()},
 * {@link HttpServletRequest#getScheme() getScheme()},
 * {@link HttpServletRequest#isSecure() isSecure()}, and
 * {@link HttpServletResponse#sendRedirect(String) sendRedirect(String)}.
 * In effect the wrapped request and response reflect the client-originated
 * protocol and address.
 *
 * <p><strong>Note:</strong> This filter can also be used in a
 * {@link #setRemoveOnly removeOnly} mode where "Forwarded" and "X-Forwarded-*"
 * headers are only eliminated without being used.
 *
 * @author Rossen Stoyanchev
 * @author Eddú Meléndez
 * @author Rob Winch
 * @see <a href="https://tools.ietf.org/html/rfc7239">https://tools.ietf.org/html/rfc7239</a>
 * @since 4.3
 */
public class ForwardedHeaderFilter extends OncePerRequestFilter {

    private static final Set<String> FORWARDED_HEADER_NAMES =
            Collections.newSetFromMap(new LinkedCaseInsensitiveMap<>(5, Locale.ENGLISH));

    static {
        FORWARDED_HEADER_NAMES.add("Forwarded");
        FORWARDED_HEADER_NAMES.add("X-Forwarded-Host");
        FORWARDED_HEADER_NAMES.add("X-Forwarded-Port");
        FORWARDED_HEADER_NAMES.add("X-Forwarded-Proto");
        FORWARDED_HEADER_NAMES.add("X-Forwarded-Prefix");
    }


    private final UrlPathHelper pathHelper;

    private boolean removeOnly;

    private boolean relativeRedirects;


    public ForwardedHeaderFilter() {
        this.pathHelper = new UrlPathHelper();
        this.pathHelper.setUrlDecode(false);
        this.pathHelper.setRemoveSemicolonContent(false);
    }


    /**
     * Enables mode in which any "Forwarded" or "X-Forwarded-*" headers are
     * removed only and the information in them ignored.
     *
     * @param removeOnly whether to discard and ignore forwarded headers
     * @since 4.3.9
     */
    public void setRemoveOnly(boolean removeOnly) {
        this.removeOnly = removeOnly;
    }

    /**
     * Use this property to enable relative redirects as explained in
     * {@link RelativeRedirectFilter}, and also using the same response wrapper
     * as that filter does, or if both are configured, only one will wrap.
     * <p>By default, if this property is set to false, in which case calls to
     * {@link HttpServletResponse#sendRedirect(String)} are overridden in order
     * to turn relative into absolute URLs, also taking into account forwarded
     * headers.
     *
     * @param relativeRedirects whether to use relative redirects
     * @since 4.3.10
     */
    public void setRelativeRedirects(boolean relativeRedirects) {
        this.relativeRedirects = relativeRedirects;
    }


    @Override
    protected boolean shouldNotFilter(HttpServletRequest request) throws ServletException {
        for (String headerName : FORWARDED_HEADER_NAMES) {
            if (request.getHeader(headerName) != null) {
                return false;
            }
        }
        return true;
    }

    @Override
    protected boolean shouldNotFilterAsyncDispatch() {
        return false;
    }

    @Override
    protected boolean shouldNotFilterErrorDispatch() {
        return false;
    }

    @Override
    protected void doFilterInternal(HttpServletRequest request, HttpServletResponse response,
                                    FilterChain filterChain) throws ServletException, IOException {

        if (this.removeOnly) {
            ForwardedHeaderRemovingRequest theRequest = new ForwardedHeaderRemovingRequest(request);
            filterChain.doFilter(theRequest, response);
        } else {
            HttpServletRequest theRequest = new ForwardedHeaderExtractingRequest(request, this.pathHelper);
            HttpServletResponse theResponse = (this.relativeRedirects ?
                    RelativeRedirectResponseWrapper.wrapIfNecessary(response, HttpStatus.SEE_OTHER) :
                    new ForwardedHeaderExtractingResponse(response, theRequest));
            filterChain.doFilter(theRequest, theResponse);
        }
    }


    /**
     * Hide "Forwarded" or "X-Forwarded-*" headers.
     */
    private static class ForwardedHeaderRemovingRequest extends HttpServletRequestWrapper {

        private final Map<String, List<String>> headers;

        public ForwardedHeaderRemovingRequest(HttpServletRequest request) {
            super(request);
            this.headers = initHeaders(request);
        }

        private static Map<String, List<String>> initHeaders(HttpServletRequest request) {
            Map<String, List<String>> headers = new LinkedCaseInsensitiveMap<>(Locale.ENGLISH);
            Enumeration<String> names = request.getHeaderNames();
            while (names.hasMoreElements()) {
                String name = names.nextElement();
                if (!FORWARDED_HEADER_NAMES.contains(name)) {
                    headers.put(name, Collections.list(request.getHeaders(name)));
                }
            }
            return headers;
        }

        // Override header accessors to not expose forwarded headers

        @Override
        @Nullable
        public String getHeader(String name) {
            List<String> value = this.headers.get(name);
            return (CollectionUtils.isEmpty(value) ? null : value.get(0));
        }

        @Override
        public Enumeration<String> getHeaders(String name) {
            List<String> value = this.headers.get(name);
            return (Collections.enumeration(value != null ? value : Collections.emptySet()));
        }

        @Override
        public Enumeration<String> getHeaderNames() {
            return Collections.enumeration(this.headers.keySet());
        }
    }


    /**
     * Extract and use "Forwarded" or "X-Forwarded-*" headers.
     */
    private static class ForwardedHeaderExtractingRequest extends ForwardedHeaderRemovingRequest {

        @Nullable
        private final String scheme;

        private final boolean secure;

        @Nullable
        private final String host;

        private final int port;

        private final String contextPath;

        private final String requestUri;

        private final String requestUrl;

        public ForwardedHeaderExtractingRequest(HttpServletRequest request, UrlPathHelper pathHelper) {
            super(request);

            HttpRequest httpRequest = new ServletServerHttpRequest(request);
            UriComponents uriComponents = UriComponentsBuilder.fromHttpRequest(httpRequest).build();
            int port = uriComponents.getPort();

            this.scheme = uriComponents.getScheme();
            this.secure = "https".equals(scheme);
            this.host = uriComponents.getHost();
            this.port = (port == -1 ? (this.secure ? 443 : 80) : port);

            String prefix = getForwardedPrefix(request);
            this.contextPath = (prefix != null ? prefix : request.getContextPath());
            this.requestUri = this.contextPath + pathHelper.getPathWithinApplication(request);
            this.requestUrl = this.scheme + "://" + this.host + (port == -1 ? "" : ":" + port) + this.requestUri;
        }

        @Nullable
        private static String getForwardedPrefix(HttpServletRequest request) {
            String prefix = null;
            Enumeration<String> names = request.getHeaderNames();
            while (names.hasMoreElements()) {
                String name = names.nextElement();
                if ("X-Forwarded-Prefix".equalsIgnoreCase(name)) {
                    prefix = request.getHeader(name);
                }
            }
            if (prefix != null) {
                while (prefix.endsWith("/")) {
                    prefix = prefix.substring(0, prefix.length() - 1);
                }
            }
            return prefix;
        }

        @Override
        @Nullable
        public String getScheme() {
            return this.scheme;
        }

        @Override
        @Nullable
        public String getServerName() {
            return this.host;
        }

        @Override
        public int getServerPort() {
            return this.port;
        }

        @Override
        public boolean isSecure() {
            return this.secure;
        }

        @Override
        public String getContextPath() {
            return this.contextPath;
        }

        @Override
        public String getRequestURI() {
            return this.requestUri;
        }

        @Override
        public StringBuffer getRequestURL() {
            return new StringBuffer(this.requestUrl);
        }
    }


    private static class ForwardedHeaderExtractingResponse extends HttpServletResponseWrapper {

        private static final String FOLDER_SEPARATOR = "/";

        private final HttpServletRequest request;

        public ForwardedHeaderExtractingResponse(HttpServletResponse response, HttpServletRequest request) {
            super(response);
            this.request = request;
        }

        @Override
        public void sendRedirect(String location) throws IOException {

            UriComponentsBuilder builder = UriComponentsBuilder.fromUriString(location);
            UriComponents uriComponents = builder.build();

            // Absolute location
            if (uriComponents.getScheme() != null) {
                super.sendRedirect(location);
                return;
            }

            // Network-path reference
            if (location.startsWith("//")) {
                String scheme = this.request.getScheme();
                super.sendRedirect(builder.scheme(scheme).toUriString());
                return;
            }

            String path = uriComponents.getPath();
            if (path != null) {
                // Relative to Servlet container root or to current request
                path = (path.startsWith(FOLDER_SEPARATOR) ? path :
                        StringUtils.applyRelativePath(this.request.getRequestURI(), path));
            }

            String result = UriComponentsBuilder
                    .fromHttpRequest(new ServletServerHttpRequest(this.request))
                    .replacePath(path)
                    .replaceQuery(uriComponents.getQuery())
                    .fragment(uriComponents.getFragment())
                    .build().normalize().toUriString();

            super.sendRedirect(result);
        }
    }

}
